The whole length and breadth of the Colca Canyon is an open book in whose pages -the outcropping rocks eroded through thousands of centuries- we can read the history of the earth in this part of the American continent.
An examination of the slopes of the Colca River, including its lower section in the south-west (which becomes the Majes River, and later as it flows into the sea the Camaná River, which makes a total of approximately 400 km from the source in a 4,500 m mountain range to its outlet into the Pacific Ocean), gives evidence of geological formations existing since the most ancient eras (gneis, pre-Paleozoic), to the most recent (Quaternary lava flows and recent aluvial and eluvial products).
To here explore this fascinating subject in detail is beyond our purpose. It will be sufficient to present a summary which will be more accessible to lovers of nature everywhere, and especially to those who have the opportunity to tour the region which the impressive and spectacular Colca - Majes - Carnana system cuts through.
The development of the Colca's riverbed, from its birthplace in the heights of Imata (4,400 m above sea level) located in the area of Lake Jayuchaca or Indio is capricious. The Southern Railway runs through precisely this area and one can see from the train the plateau which divides the valley cut by the river from the valley which runs toward Lake Titicaca, including the two beautiful lacustrion mirrors of Lagunillas and Saracocha.
Two streams are born in the vicinity of the railroad station of Crucero Alto (4,420 m), and farther to the south other branches flow down from the heights of Soracancha, Irachoqui (4,600 m) and Antajahua (4,780 m). It is probably here that we find the most remote branch of the Chilamayo, and thus the true beginning of the Colca River. From this point the stream flows to the north-east in a wide channel with quiet waters for approximately 70 km, arriving near Huinco (3,950 m). Here it changes direction abruptly and flows toward the south to Callalli (3,867 m). By this time, with the river now at 3,800 m and at 3 km from the village, it has covered another 25 km.
The terraces are of pluvial - lacustrian origen and were abandoned by the river because of its channeling subsequent to the orogenic elevation of the Andes Mountain Range during the last geological eras: the Tertiary or Cenozoic and the Quaternary or Neozoic. These terraces have been ingeniously taken advantage of for cultivation terraces, surprising in their perfection, although they are also a cause for concern due to the many rock slides toward the channel of the river resulting from the irrigation of the little farms.
After the waters of the Choco join the Colca, the river takes a south-southwest direction, still deep within its canyon walls, with gradients between 40 and 70 m./km, and passes the village of Huambo at a distance. The village lies on a terrace to the left of the river at an altitude of 3,332 m, while the bed of the river is at 1,490 m; the distance between them is 15 km. Farther on, in the Valley of the Volcanos, the Colca receives the waters of the river Andagua, or Mamacocha, which is important during the dry periods as a natural regulation of the water level of the lake of the same name which lies a few kilometers from the confluence.
There are about 7 km of channel from Andamayo to the beginning of the Hacienda Peril. From this point to the spur of coastal batholith where we find the Jaraba Hacienda at 208 m, and where the river again digs into a deep canyon when it finds itself obliged to take a wide curve to the right, spread some 55 km more along the fruitful Majes Valley.
Crossing the batholith litoral, to finally flow out into the sea at the beaches of Camaná adds another 48 km of journey to the Colca, which makes a total of some 400 km of length for the complete Colca-Majes-Camaná system. Peñaherrera, in his General Geography of Perú Volume 1, 1969, attributes some 450 km to the length of the river. This is the probable length of the total channel if we take into account the many curves and the longest branch, not accurately measured, from its birth at the foot of the peak of Antajahua.
Morphology of the canyon
In the foregoing section we have seen that the Colca descends from 4,500 m of altitude to 0 m at Camaná in a journey of approximately 450 km. This is a striking difference in elevation, remarkably exaggerated as far as Andamayo, which has been the cause of the deep erosive action of the waters between this spot and the tributary streams higher up, that is to say to the east of the north west/south-east alignment of the long chain of giant volcanos, represented here by Coropuna (6,425 m) on the right of the Colca and Ampato (6,300 m) on the left.
In reality this river, as the other great rivers to the north and south, crosses in its journey three well defined types of morphology. We are referring to three parallel belts which run from north-west to south-east: that of the occidental watershed of the Andes Range, with its rosary of great volcanos; that of the costal deserts, arid pampas which are the northern projection of the Atacama Desert (Bowman); and that of the hills of the cristalline costal massif, as the projection of the Paracas Peninsula.
The formation of the canyon in the area of the great volcanos on the occidental watershed of the Andes is extraordinary, with characteristics which allow us to determine that we are confronting one of the deepest on the earth. The bed of the Colca lies between the canyon walls at an altitude of 1,380 m, so that between the average altitude of the surrounding volcanic peaks (6,362 m) and the channel there is a real difference in elevation of 4,982 m.
The channeling of all the rivers which flow down from the Andes to the Pacific Ocean, such as those which descend to the Amazon valley, are characterized by canyons. In order to explain this geomorphologic phenomenon it is necessary to make a brief orogenic summary starting with the Cretaceous period (upper Mesozoic, 70 million years ago) when the first Andean elevation occurred, known as the Peruvian Folding (Steinmann), with which the marine stage of the geological development of the continent ended.
This great orogenic movement was followed by abundant lava flows which spilled over the occidental watershed of the southern range, forming a massive platform on which long after (at the end of the Tertiary) the great volcanoes which extend from Sara-Sara (5,800 m) at 15 degrees southern latitude to Tierra del Fuego, south of Chile, began to grow. Almost at the same time that these flows were produced, intrusions of igneous rock (crystalline) occurred which, after a long process of atmospheric erosion, were exposed to the light of day forming the extraordinary ranges of Huaraz, with the peak of Huascaran (6,768 m); of Vilcabamba, with Salcantay (6,360 m); of Urubamba with Ausangate (6,248 m), and others.
In the Oligocene (middle Tertiary, 35 million years ago) the true emersion of the Continental Range occurred with a series of phenomena which produced an elevation between 1000 and 2000 meters above sea level, giving rise to new foldings in the sedimentary strata. This orogeny has been named the Incan Folding Steinmann. At the end of the Tertiary (the Miocene and Pliocene) between 2 and 30 million years ago, new eruptions were produced which caused a great accumulation of volcanic layers. New crystalline intrusions also occurred which were the origin of the coastal batholiths, such as that of Caldera near the city of Arequipa (on the road to Mollendo).
Tangential thrusts, especially in the Pliocene (upper Tertiary, 15 million years ago), known as the Quichuano Folding (Steinmann), brought the range to the altitude we know today. Between the Pliocene and the Pleistocene (Quaternary, 2 million years ago) volcanic activity began again and gave birth to the great masses which adorn the southern territory like a rosary, and among which we find Coropuna and Ampato.
We can see from this rough outline that due to the vertical and tangential east west thrusts which the whole region underwent (according to the theory of Plate Tectonics widely accepted in recent years) the range rose in stages with great intervals of time (millions of years) between each movement.
It is thus that we can explain the powerful erosive effect of the waters which flowed down from the glaciers and the different glaciations which occurred on the peaks of the range millions of years ago, and also that of the flooding rains of today, which, with their repeating cycles, give us cause for concern at the damage they cause.
Evidence for the rises which were produced during the process of the emergence of the range can be found in the pluvial - lacustrian terraces which have been left at a higher altitude than the present river bed. A tourist traveling through the valley can discover it for himself in the wide, intensively cultivated terraces in the area of Chivay and Cabanaconde on both sides of the canyon, and even at Huambo and beyond, in the same way we can explain the elevation of the marine beaches which have been formed into terraces at the ocean on the occidental slopes of the continent at Camaná and especially at Chala where one can count as many as four terraces.
It is also easy to observe the effects of the erosive action of the glaciers of past times which have modeled the landscape of the high, bleak region of the range, from 4,000 m of altitude, leaving more or less gentle ridges the length and breadth of the area covered by the different tributaries which flow together to feed the river basin. it is surprising that we do not find more traces of glacial deposits. But these are scarce, since the erosion of the steep slopes has caused them to be carried down the valley to the sea, or at least as far as the great trough of the Majes Valley.
In short, the break with the base level due to the repeated elevations of the Andes (in terms of geological time) is responsible for the cutting of the deep canyons by the rivers and the formation of the terraces on both sides of the range. This is the origin of the impressive canyons which so strongly attract tourists to this area from all over the world.
We will complete this picture of the region watered by the Colca-Majes-Camaná basin with some references to the geological history of' the tellural outcroppings which make up the formations found along the length of the system.
From its birth place in the snowy heights to the river's mouth at the Pacific, and along both flanks, we can distinguish rock strata distributed in succession: the oldest at the bottom, the later above them and the most recent on top, flush with the soil. There are, however, some places where the tectonic compression has so convulsed the layers that they appear in the reverse order. But this is not the case in the Colca basin. We also find intrusions of magma derived from crystalline rocks in the form of batholiths and dikes which cut across the pre-existing formations.
The stratographic sequence turns out to be an open book to the observer who can read the history of the earth, starting with the oldest formations, which are found in the cut the Majes River has made in the zone of the coastal range before pouring out into the wide valley of Camaná Various pages are missing from this book, namely those which were either not deposited, or were removed because of their exposure to the forces of erosion. Thus we can say that from the whole Paleozoic (Primary) era four periods are missing: the Cambrian, the Silurian, the Carboniferous and the Permian. In the Mezozoic (Secondary) we lack the Triassic period.
The aforementioned cut in the coastal massif is a deep canyon 40 km long. in it rocks outcrop from the basal complex of the coast which date from the Precambrian era (more than 500 million years ago). In reality it is an example of the primitive crystalline base of the earth's crust.
Already in the plateau region thick volcanic layers appear which cover all the underlying formations. Especially evident are basalts and andesites (Barroso formation) so old that they are considered igneous (Sencia formation) because of the metamorphic process they have suffered. The megaliths spilled from the heights toward the valley provided the ancient aborigines with surfaces for carving the amazing petroglyphs of Toro Muerto near Corire.
The tirany of space does not permit us to deal with our subject more extensively. Briefly, coming up from the Majes Valley and entering the narrows of the Colca Canyon proper, we find sandstones, limestones, slates and quartzites alternating in the almost vertical scarps, which belong to the Jurasic and Cretaceous. We see here strata of anticlinal and sinclinal folds.
Nonetheless, along the dividing line of the range, between the Colca and Callalli, at an altitude of 4,600 m we find outcroppings of quartzites of the Yura formation and Arcurquina limestones Jurassic and Cretaceous) which give evidence of the amazing orogenic thrusts which carried to such an elevation these ancient sedimentary formations.
The materials carried by erosion and left as deposites on the plains with less than 20' of inclination belong to the recent Quaternary (Holocene). A classic example is the Majes Valley which turns out to be an aluvial deposit of great magnitude. Thanks to the patient and sacrificing labor of many generations of farmers, splendid fields, richly cultivated and crossed by irrigation ditches bordered by willows and other trees, which give freshness and pleasant shade to the inhabitants, have been developed.
In rough strokes this is the geomorphological panorama unveiled by the erosion of the torrential Colca during many millennia as it agressively flows down from the immaculate heights of the distant peaks, at nearly 5,000 m of altitude, to the solemn Pacific Ocean, on its journey of nearly 450 km, The entire region is a delight to the eve, and thus to the most demanding spirit.
In conclusion, we do not feel we have dealt in too much detail with the geornorphological panorama of a region of Perú so complex and interesting, but rather that we have erred in the other direction in having to omit many details, for which the specialist must forgive us.